Packaged microelectronic assemblies, such as memory chips and microprocessor chips, typically include a microelectronic die mounted to a substrate and encased in a plastic protective covering. The die includes functional features, such as memory cells, processor circuits, and interconnecting circuitry. The die also typically includes bond pads that are electrically coupled to the functional features. The bond pads are coupled to pins or other types of terminals that extend outside the protective covering for connecting the dies to buses, circuits, and/or microelectronic assemblies. Alternatively, bare microelectronic dies can be connected to other microelectronic assemblies.
Multiple dies are typically formed simultaneously on a single microfeature workpiece (e.g., a wafer), and are electrically tested at the wafer level prior to being singulated and packaged. Probe devices are typically used to perform the electrical test. Existing probe devices can include a chuck that supports the wafer, and a probe card positioned above the chuck and the wafer. The probe card includes a large number of downwardly extending pins that make physical and electrical contact with bond pads on each die when the chuck moves in an upward direction. Once electrical contact is established between the probe card and the wafer-level dies, each die is electrically tested. The dies are then singulated, and acceptable dies are packaged while defective dies are discarded or reworked.
One difficulty with existing probe devices is that not all wafers necessarily have the same thickness. Accordingly, it may be difficult to determine how far to move the wafer in an upward direction so as to make electrical contact with the pins, without crushing the pins. One approach to solving this problem has been to use pneumatic proximity sensors to sample the upper surface of the chuck (e.g., at five to seven locations) and determine the location of the chuck upper surface based on the average of the sampled values. Then a wafer is placed on the chuck, drawn tightly against the chuck with vacuum, and the same sensors are used to determine the position of the wafer upper surface. The thickness of the wafer can be determined based on the difference between the location of the wafer upper surface and the chuck upper surface. This value can then be used to determine how far to move the wafer in an upward direction (relative to a known position of the chuck) in order to make contact with the pins. Suitable devices for electrically testing the wafer and determining the wafer thicknesses are available from Accretech USA, Inc. of Bloomfield Hills, Mich. (model numbers APM90, UF200 and UF3000) and Olympic Systems, Inc. of Rocklin, Calif. (Electroglas model 4090).
While the foregoing approach has been suitable for determining the position to which the wafer is elevated during testing, this technique has nevertheless resulted in pins being crushed. Once a pin is crushed, the entire card is generally rendered inoperative and must be discarded. Because the card may include over 10,000 pins, along with associated test circuitry, the cards are typically high-value items and therefore the loss of a card can significantly increase the cost of producing microelectronic die packages.